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Calibrating an optical scanner for quality assurance of large area radiation detectors
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2014 Meas. Sci. Technol. 25 115403
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Measurement Science and Technology
Meas. Sci. Technol. 25 (2014) 115403 (11pp)
doi:10.1088/0957-0233/25/11/115403
Calibrating an optical scanner for quality
assurance of large area radiation detectors
A Karadzhinova1, T Hildén1,2, M Berdova3, R Lauhakangas1, J Heino1,
E Tuominen1, S Franssila3, E Hæggström2 and I Kassamakov1,2
1
Detector Laboratory, Helsinki Institute of Physics, PO Box 64, FI-00014, Helsinki, Finland
Department of Physics, University of Helsinki, PO Box 64, FI-00014, Helsinki, Finland
3
Department of Material Science and Engineering, Aalto University and Micronova Nanofabrication
Centre, PO Box 17800, FI-0076, Espoo, Finland
2
E-mail: [email protected]
Received 11 April 2014, revised 15 July 2014
Accepted for publication 28 August 2014
Published 13 October 2014
Abstract
A gas electron multiplier (GEM) is a particle detector used in high-energy physics. Its main
component is a thin copper-polymer-copper sandwich that carries Ø =70 ± 5 µm holes. Quality
assurance (QA) is needed to guarantee both long operating life and reading fidelity of the GEM.
Absence of layer defects and conformity of the holes to specifications is important. Both hole
size and shape influence the detector’s gas multiplication factor and hence affect the collected
data. For the scanner the required lateral measurement tolerance is ± 5 µm. We calibrated a high
aspect ratio optical scanning system (OSS) to allow ensuring the quality of large GEM foils.
For the calibration we microfabricated transfer standards, which were imaged with the OSS
and which were compared to corresponding scanning electron microscopy (SEM) images. The
calibration fulfilled the ISO/IEC 17025 and UKAS M3003 requirements: the calibration factor
was 1.01 ± 0.01, determined at 95% confidence level across a 950 × 950 mm2 area. The proposed
large-scale scanning technique can potentially be valuable in other microfabricated products too.
Keywords: calibration, optical, transfer standard, expanded uncertainty
S Online supplementary data available from stacks.iop.org/MST/11/115403/mmedia
(Some figures may appear in colour only in the online journal)
1. Introduction
feature a 140 µm pitch [2]. Little is known about aging of GEM
detectors under harsh radiation conditions4. Detecting defects,
for example holes blocked or deformed by the etching process,
are important for verifying foil usability. The electric field shape
inside the detector makes each hole act as an electron multiplier through an avalanche process [2]. The size and shape of
the hole influence the gas multiplication factor [5]. A factor of
104–105 can be reached by stacking several GEM foils. This
charge amplification technique has a 50 MHz mm−2 rate capability, which is restricted by ion build up. The technique permits
Particle and nuclear detectors used in contemporary highenergy physics experiments require precisely engineered
structures. The detectors should be maintenance free since
devices cannot usually be replaced during their lifespan.
Longevity in a severe radiation environment should therefore
be guaranteed. Quality assurance (QA) may guarantee long
operating life of the detector [1].
Gas electron multiplier (GEM) detectors [2–4] are used in
the European Organization for Nuclear Research (CERN), in
the Facility for Antiproton and Ion Research (FAIR) and in the
Joint European Torus (JET). A GEM detector features a densely
pierced 50 µm thick polymer foil, coated with a 5 µm thin copper
layer on both sides. The holes in the GEM foil, figure 1, have
a diameter of 70 ± 5 µm (outer) and 50 ± 5 µm (inner) and
0957-0233/14/115403+11$33.00
4
A study related to GEM aging done by the CERN/COMPASS experiment
is from 2012. The observed aging was supposed to be due to the Silicon:
Dow Corning 1-2577 (R2SiO.) and Sulphur: Sulphuric Acid (H2SO4) coating
on the GEM foil surface. The GEMs were operated in Ar–CO2 (70:30) at an
effective gas multiplication factor of 8.5 × 103. More information about the
GEM foil aging is expected after the CERN/TOTEM GEM detectors autopsy.
1
© 2014 IOP Publishing Ltd Printed in the UK
A Karadzhinova et al
Meas. Sci. Technol. 25 (2014) 115403
Figure 1. Scanning electron microscopy image of GEM foil—top view (left) and cross section (right).
detecting the presence and position of charged particles, photons, x-rays and neutrons [2]. An absolute change of 1 µm in hole
diameter alters the amplification by 2% and bandwidth by 5%.
Thus a ± 5 µm tolerance (10 µm total uncertainty) corresponding
to 1 dB (20%) amplitude amplification uncertainty requires that
any calibrated quality assurance tool should resolve 1 µm. These
calculations are done based on results presented in [5].
The Optical Scanning System [6] (OSS) was developed for
quality assurance of GEM foils. According to technical design
review documents being prepared: since the proposed FAIR
and CERN experiments require an estimated 340 m2 of GEM
foil and since the area hole density is 6400 holes per cm2, the
quality assurance should be able to deal with 22 billion holes in a
­limited time. This translates into a targeted inspection rate of ca.
5 × 106–10 × 106 holes per day with 5 µm imaging resolution.
Optical quality control, being rapid and non-contacting, can
in principle address this challenge during manufacturing and
assembly [1]. The current OSS set-up features 1.73 µm pixel
size, 950 × 950 mm2 scan area and a projected inspection rate
of 5 × 106–10 × 106 holes per day5. We are currently taking
several images that are stitched into one [7] to allow large-scale
QA of micron size futures. However, this requires calibrated
measurements and quality assurance tools.
For lateral calibration three different designs of microfabricated transfer standards (TS) were produced. In this paper we
focus on TS#2. The TS#1 results are reported in [8].
Figure 2. Three different transfer standard designs: (a) TS#1, (b)
TS#2 and (c) TS#3.
•The TS should permit calibration of a single cavity image
(calibrated microscopy) as well as calibration of the
cavity matrix image (calibrated landscape surveying).
• Non-destructive methods must be used for TS calibration.
•The calibration procedure must be traceable to standards.
We microfabricated the TS [9] since this established manufacturing technology can produce patterns simulating the holes
and pierced matrix in the GEM foil [2]. It also provides standards with similar optical reflectance as that of the GEM foil.
This is important since the calibration depends on the surface
properties of the sample [10], see section 2.2.
The holes in the GEM foil are chemically etched from both
sides of a 60 µm (including metallization) thick copper–­Kapton–
copper film, which results in double-cone shaped holes, figure 1.
For OSS calibration three different kinds of TS were designed
and manufactured. A 300 µm thick silicon wafer served as
starting material. In TS#1 the cavity represents the outer diameter D of the GEM holes, figure 2(a). In figures 2(b) and (c),
respectively, the TS cavity represents both the outer and the inner
diameters (D, d) of the GEM hole; TS#2 is used to calibrate the
OSS when the standard is illuminated from above, whereas TS#3
is used when the standard is illuminated both from both above
and below. Each TS was processed to feature a uniform matrix
of 4 × 100 cavities with correct D, d and P, to obtain statistics
that describes the OSS performance during small area scanning.
2. Methods
2.1. Design and manufacturing of transfer standards
2.1.1. Design. The standard should allow calibrating the OSS
to achieve 5 µm imaging accuracy across 1 m2 at 500 mm s−1
maximum scan speed. strict requirements were applied to the
TS design.
•The TS layout should replicate the hole pattern in the
GEM foil, the hole size, the pitch (P) between the hole
centres and the rim roughness of the inner and outer
diameter (d and D).
2.1.2. Manufacture. The TS fabrication was done at the
Micronova Nanofabrication Centre, Finland, using established microfabrication techniques [11], figure 3. To produce
5
This is based on our current work where most scanned areas were
10 × 10 cm2 foils with 640 000 holes that were scanned in 1 h. By scanning
8 h a day, the inspection rate is 5 million holes per day.
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Meas. Sci. Technol. 25 (2014) 115403
Figure 4. Manufacturing process for TS#2 and TS#3.
Figure 3. Process flow for TS#1: (a) cleaning of Si substrate, (b)
20 nm of ALD Al2O3, (c) laser writing, (d) BHF etching, (e) RIE of
Si and (f) removal of Al2O3 by BHF.
TS#1, first a 20 nm thick atomic layer deposited (ALD) Al2O3
film was grown on a clean Si substrate, figures 3(a) and (b),
in a Beneq TFS-500 ALD reactor. Pressure was 10 mbar and
deposition temperature 220 °C. Precursor gases were trimethylaluminum (AlMe3 a.k.a. TMA) and water. Each deposition
cycle comprised of a 0.25 s AlMe3 pulse, followed by a 0.75 s
N2 purge, 0.2 s H2O pulse and a 0.75 s N2 purge pulse. The
cavity pattern was exposed by laser lithography. The photoresist (AZ5214E, Microchemicals GmbH) was exposed using
a 405 nm wavelength of a semiconductor laser (LW405-A
LaserWriter, Microtech), figure 3(c). Aluminum oxide was
wet etched in buffered hydrofluoric acid (BHF) (Honeywell)
for 20 s, figure 3(d). Subsequently, 500 nm deep cavities
were etched in silicon by reactive ion etching (RIE) in SF6/
O2 plasma (SF6 100 sccm, O2 5 sccm, 30 mTorr, 100 W) in an
Oxford Plasmalab 80 system. Finally Al2O3 was etched away
by BHF, figure 3(e) and the final TS#1 is shown in 3(f).
Figure 4 shows the fabrication processes for TS#2 and
TS#3. First the double side polished (DSP, 300 µm) wafer was
cleaned in hydrofluoric acid, figure 4(a). Then AZ5214E photoresist was spin coated on the wafer, figure 4(b). Photoresist
was patterned by laser lithography as above. This resist served
as an etch mask in ICP-RIE cryo-etching of 70 µm diameter
shallow holes (SF6 40 sccm, O2 6 sccm, at −120 °C, 800 W
ICP power and 3 W CCP power), figure 4(d). Photoresist
removal was done in acetone and isopropanol, after which
20 nm of Al2O3 was deposited using ALD. Another laser
lithography step with AZ5214E photoresist, figure 4(f) and
etching of Al2O3 in BHF, figure 4(g) was performed to define
50 µm diameter holes. This Al2O3 layer served as a hard
mask for 50 µm deep silicon etching. This etching was done
using a Bosch process with fluorine (SF6), oxygen (O2) and
octafluorocyclobutane (C4F8), figure 4(h1). To fabricate the
TS#3 standards, the silicon etching (300 µm through wafer)
was also done using a Bosch process with the same gas
mixture, figure 4(h2). In the end all Al2O3 was removed by
BHF wet etching.
Figure 5. Optical Scanning System in the clean room of the
Detector laboratory at Helsinki Institute of Physics and University
of Helsinki.
Figure 6. Image of GEM foil taken with the OSS.
2.2. Optical scanning system
The OSS for quality control of GEM foils, figures 5 and
6, is a high-resolution imaging instrument suitable for rapidly scanning large areas (15 images min−1 stitched together
[7]). The movement across the scan area is done with a high
rigidity direct actuator model SMC LJ1 [12]. The positioning
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Figure 7. Images of GEM foil taken with OSS using in-line illumination. Matt surface foil with strong diffuse properties (Left) and mirror
surface foil with strong specular properties (Right) (III.R1.A.3.).
Figure 8. Refractive index (n) and extinction coefficient (k) as a function of illumination wavelength for copper (left) and silicon wafer
(right) [19, 20]. The wavelengths used by the OSS are indicated in blue.
repeatability along the XY direction is ±100 µm for more
than one meter of travel at 500 mm s−1. To vertically adjust
the camera focus there is an SMC LTF series rolled ball
screw [12] with ±50 µm positioning repeatability. The 9.0
Mpix Artray [13] Artcam-900MI color CMOS camera,
1.0 × in-line illumination telecentric C-mount lens package
(figure 5) from Edmund Optics [14] and dc-950 H fibreoptic illuminator from Dolan–Jenner [15] provides 1.73 µm
pixel size. The OSS objective with its in-line illumination
moves together along the Z axis with the translation stage.
The diameter of the guide light is 7.9 mm (outer diameter)
and 4.3 mm (inner diameter—the active area) and the objective’s field of view is 6.4 × 4.8 mm2. The fluence is chosen
for each sample such that it maximizes the dynamic range
of the camera.
A custom made LabView [16] program that is time synchronized to the operating system controls the camera motion and
monitors the imaging. We are still working on implementing
true on-the-fly imaging (currently we do stop-and-go).
For each imaging position the need for auto focusing is
determined by the value obtained with the focus algorithm
[17]. If this value is below a certain threshold, then auto
focusing is needed. In practice auto focusing is carried out by
defining the ‘best focus’, which is the highest value obtained
by the focusing algorithm.
Using in-line illumination, the rays are reflected and scattered away from the lens in such a manner that none of the
light that hits the object is reflected back into the lens and
onto the sensor. That improves the edge contrast and makes
it easier to identify the holes and cavity edges. The Dolan–
Jenner light source uses an OSRAM Sylvania 54842 EKE
Figure 9. Darker and lighter areas observed during GEM foil
scanning.
lamp (21 V, 150 W) and a color filter kit with optical transmission in 470–660 nm. The reflectivity of the GEM foil
surface varies between individual foils, see figure 7 [18].
The left image in figure 7 is from a GEM foil with a matt
surface whereas the right one is from a mirror surface. They
are taken with the same scan parameters.
Figure 8 shows the index of copper [19] (left) and Si wafer
[20] (right); the refractive index of Kapton is 1.70.
Several issues affect the image: homogeneity of illumination; sample tilt, waviness and position in the light field
(focus), surface reflectivity and roughness, rim roughness
(D and d), conical wall tilt. These all affect the precision and
accuracy of the optical measurement. Sample waviness along
the Z coordinate of the foil surface (due to waviness, wrinkles
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Figure 10. Image of FAIR GEM foils to be inspected. Alignment issues with the masks make the holes elliptical. The Kapton (black half
circles) is visible only for one side of the hole. This is not a shadow effect.
etc) cause a lighter/darker spot in the protruding/indented
area, as illustrated in figure 9. This affects either part of the
image or all of it and hence either makes the image hard to
analyze or altogether unsuitable for analysis.
This variation is often caused by scratches on the foil surface, oxidation, chemical residue from the manufacture or dust
particles. The TS reflectivity, (figure 8 (right)) differs from that
of the GEM foil, since they are made from different materials.
Moreover, the polished TS surface is flatter (surface roughness
average—0.18 nm) than the naturally wavy GEM foil (surface
roughness average—360 nm), which affects the apparent hole
size. This is due to the fact that too much light on the TS cavity
edge causes glare, which makes it hard to determine D since
the edge appears thinner than it is. Too little light on the other
hand makes d appear too small since it is hard to see the edge
of the inner cavity. The same is true for the GEM foil. Using
different filters and light intensity helps in detecting the edge
of the hole. To accurately measure D, a 400 nm day light filter
[21] and appropriate (for the camera) light intensity were used.
By controlling the light source emission we control the
light intensity on the sample surface. Together with the
autofocus capacity this makes estimating both D and d
easier. In practice these parameters can be defined by any
straight-line segment that passes through the centre of the
hole and whose endpoints lie on the edge of the hole. This
approach assumes circular holes. There are GEM foils with
defects or elliptical holes due to mismatch (figure 10) of the
double masks used during fabrication. These holes cannot
be analyzed with the simple approach. This study does not
focus on the ellipticity of the holes and mask mismatch.
In practice this means that ability to maintain autofocus onthe-fly across a large scan area is a crucial feature that needs
to be implemented (automatic control of light intensity for onthe-fly operation may be possible whereas it is hard to implement filter switching for on-the-fly operation).
Figure 11. SIRA SEM calibration specimen S170. The
19.7 lines mm−1 with period = 50.8 µm has ± 0.02 µm expanded
uncertainty.
determined using a SIRA SEM calibration specimen S170
[23], figure 11. The calibration grid was measured 3 times,
both vertically and horizontally without removing the sample
between the measurements.
The combined standard uncertainty calculated for the SEM
imaging was 0.01 µm at 1000 × magnification. This value was
used in all TS measurements. The expanded uncertainty of the
SEM was ± 0.02 µm. The expanded uncertainty for one hole is
the standard uncertainty multiplied by a coverage factor k = 2,
providing a coverage probability of 95% [24].
2.4.2. Measurement method. To determine the bias in the
OSS measurement that is accounted for by the calibration
(calibration factor of OSS), each TS was divided into four
quadrants with 95–100 cavities each. Forty-five randomly
chosen cavities of TS#2 were examined by the OSS in nine
positions of the scan area (eight along the edges and one in the
centre), see figure 14.
The OSS calibration factor was derived for d and D, by
linking the SEM and OSS results using the ratio of the two
results.
For the calibration two things need to be done: (1) determine the precision and accuracy with which one can calibrate
the OSS for measuring one hole and (2) use this value together
with an ensemble measurement to derive the calibration constant for an ensemble of holes occupying a large area (one
field of view or the entire foil). The first one deals only with
issues related to imaging disparities across the field of view
whereas the latter one additionally incorporates issues related
to translating the field of view across the foil.
2.3. Gold standard
The scanning electron microscope (SEM), a Hitachi S-4800
FESEM with 3 nm lateral resolution [22], served as our ‘gold
standard’ to determine the OSS calibration factor.
2.4. Calibration and measurements procedure (one hole and
one ensemble of holes)
2.4.1. SEM calibration procedure. The Hitachi S-4800
FESEM was calibrated at 15 kV and 10 µA according to Hitachi’s specifications. The instrument expanded uncertainty was
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Figure 12. SEM image of TS cavity measurements of the d (left) and D (right).
Figure 13. TS#2 imaged by OSS (left) with the diagonal method for cavity selection (right).
In practice we relied on [24, 25] and the requirements in
section 2.1.1 and chose SEM as a base for comparison, our
gold standard. For this purpose we used a calibration specimen, figure 11, to determine the accuracy of the SEM. The
calibration grid was measured three times vertically and horizontally using the Mountains Map program’s distance measurement features [26]. We defined the SEM measurement
uncertainty according to [24].
2.4.3. SEM measurements. The chosen 44 TS#2 cavities were
scanned by SEM to determine their D and d. All cavities were
measured three times without translating the cavity between
the images. Using the Mountains Map’s features [26] the d
and D values of each cavity were measured along four different directions (see figure 12 for reference). The cavity diameter
was defined by any straight-line segment that passes through
the centre of the cavity and whose endpoints lie on the cavity
edge. The uncertainty of D and d for each cavity measurement
was defined as required in [24]. The SEM calibration specimen
measurement’s uncertainty was combined with the d and D
measurement’s uncertainty values obtained with the SEM.
All SEM measurements were done manually, using the
same Mountains Map’s features [26]. The precision and accuracy of the SEM measurements depend on identifying the
cavity edges and on the granularity of the computer mouse
movement.
Figure 14. The nine positions across the 0.9 m2 test bed used to
calibrate the OSS.
950 × 950 mm2 scan area and 1067 holes s−1 scan speed.
To verify absence of distortion—alteration of original cavity shape—and magnification errors—of a cavity imaged by
OSS, the cavities were selected close to the sample diagonals,
figure 13. The TS was placed at the centre of the nine different
2.4.4. OSS measurements. The TS#2 was imaged with
the OSS in nine randomly chosen positions inside the
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Figure 15. SEM image (Left) of transfer standard cavity TS2_3_46 with d of 53.64 ± 0.88 µm and D of 71.57 ± 0.68 µm. OSS image
(Right) of the same cavity with optically determined d = 53.06 ± 0.74 µm and D = 71.89 ± 1.03 µm.
factor is 1%. This factor is valid across 950 × 950 mm2 at
500 mm s−1 scan speed.
The calibration factor was defined using error propagation
calculus for both D and d as the ratio of the results obtained
by the two methods. This factor was applied to the OSS
results and the final results were reported and illustrated using
histograms.
Table 1. Inner (d) and outer (D) diameters of cavity TS2_3_46.
Before OSS
Feature calibration
d
D
After OSS
­calibration
SEM
53.06 ± 0.74 µm 53.56 ± 0.75 µm 53.64 ± 0.88 µm
71.89 ± 1.03 µm 71.35 ± 1.03 µm 71.57 ± 0.68 µm
This table shows the average values of TS2_3_46 (950 × 950 mm2).
positions of the table, figure 14, and scanned several times
without moving the TS between the repeats. The TS#2 fits into
a single OSS image and we made sure that it was placed at the
centre of the field of view.
Finding the perfect focus and light to scan this small object
is challenging. The naked eye doesn’t see a difference in the
focus of an image taken with e.g. Z coordinate 3850 or with
3855 (equal to a difference of 50 µm in focus distance between
the TS and the system objective). The TS#2d and D results
(obtained with the image analysis software from three or more
images) were obtained with a focus distance between the TS
and the system objective of 63 mm. The software fits ellipsoids in least-squares sense [27] to the edge contour of the
cavities. The ellipse parameters were extracted by the software. The diameter of each hole was calculated as the average
value of the major and minor axes of the ellipse. Holes with
apparent etching errors were not analyzed (figure 16).
The results from each position for each cavity (9 positions
times 45 cavities times 2 diameters = 810 measurements) are
identical for a certain focus, table position and sample region.
The d and D values and their uncertainty differ if one moves
the TS across the imaging table. The observed differences are
due to the OSS properties (imaging and translation), since the
cavities are the same. The data was recorded and the uncertainty of D and d for each cavity measurement was defined as
required in [24].
The average uncertainty in the OSS imaging was ± 0.72 µm
for d and ± 1.03 µm for D of TS#2 (2σ, n = 44 (examined
TS#2 cavities)). Two-sigma fluctuations were calculated to
show that the OSS method provides results consistent with
those provided by SEM. We compared OSS to SEM data, to
determine how close we are to the correct value (accuracy).
We also calculate the uncertainty of our estimate (precision)
at 95% confidence level. Using the OSS calibration factor, the
corresponding calibration corrections were 1.01 ± 0.01 for
d and 0.99 ± 0.01 for D. The uncertainty of the calibration
3. Results
We focus on TS#2 for system calibration. The TS#1 results
are reported in [8].
3.1. SEM
Figure 15 illustrates TS#2 cavity images, taken with SEM for
comparison with OSS images. The figure shows one cavity,
TS2_3_46, analyzed by SEM and OSS. With SEM d is
53.64 ± 0.88 µm whereas D is 71.57 ± 0.68 µm. These numbers agree with the nominal ones for the GEM foil (50 ± 5 µm
and 70 ± 5 µm).
3.2. OOS
Table 1 lists d and D for the same specific cavity TS2_3_46
obtained before and after OSS calibration. It also shows the
results obtained by SEM. The OSS results are consistent with
the SEM results.
3.3. Multiple cavity comparison
Due to a defect on the TS#2 surface one of the 45 cavities was
excluded from the calibration factor calculation, see figure 16.
Figure 17 presents the absolute difference in d measured by
SEM and OSS before and after applying the calibration factor
of 1.01 ± 0.01 (2σ). Figure 18 illustrates the Bland–Altman
test for d measured by SEM and OSS (SEM–OSS) after calibration, showing the OSS calibration range at 53–57 µm.
Figure 19 shows the Bland–Altman test for D measured
by SEM and OSS (SEM–OSS) after calibration, showing the
OSS calibration range at 70–72 µm. Both d and D measured
by OSS agree with the SEM. Figure 20 presents the absolute difference in D measured by SEM and OSS before and
after calibration with a correcting factor of 0.99 ± 0.01 (2σ).
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Meas. Sci. Technol. 25 (2014) 115403
Figure 16. The defect cavity TS2_3_25.
Figure 17. The 44 inner cavities (d) in TS#2 measured with SEM and OSS before (BC) and after (AC) calibration.
microscopy (distortion-free imaging after calibration) as well
as area surveying (distortion-free translation and imaging
after calibration). The first one is shown by figure 13 whereas
figures 18, 19 (BA plot) show that it holds within one field of
view, ca. 1.5 by 1.5 mm2. Figure 14 shows that this also holds
across large areas.
The core of this case is that one needs to maintain tight
autofocus during large fast motion because the precision and
accuracy of OSS depends on the focus.
The chosen way to show that the calibration method is
ok is valid because the bias in the OSS measurement that is
accounted for by the calibration (calibration factor of OSS),
was derived by measuring forty-five randomly chosen cavities
of TS#2 in nine positions of the large scan area, see figure 14.
For the calibration two things need to be done: (1) determine the precision and accuracy with which one can calibrate
All collected data can be found in the supplementary table
available from stacks.iop.org/MST/11/115403/mmedia.
4. Discussion
In this paper we tried to show that the OSS system can be calibrated for use with large GEM foils. To guarantee accurate
and traceable results we needed to assure that both individual
holes and ensembles of holes were correctly imaged. This
was done by employing silicon micro machined transfer standards whose dimensions were verified in a calibrated SEM
device. Our unit of comparison is the single hole whereas
the capacity to image ensembles follows from the fact that
the micro machined holes are rather identical. In practice we needed to show that the OSS performs quantitative
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Figure 18. Bland–Altman test for d measured by SEM and OSS (SEM–OSS) after calibration. All values fall inside the specified 5 µm
limit for this 1.3 × 1.4 mm2 sample.
Figure 19. Bland–Altman test for D measured by SEM and OSS (SEM–OSS) after calibration. All values fall inside the specified 5 µm
limit for this 1.3 × 1.4 mm2 sample.
OSS calibration factor was derived for d and D, by linking the
SEM and OSS results using the ratio of the two results.
The most important result is that we achieve accurate
imaging across a large TS area and by implication a large
GEM area if one can guarantee that the sandwich structure
remains as flat and as homogeneous in reflection as the silicon
TS. The diameter measurements (OSS) exhibited an uncertainty of ± 1.03 µm. These results were consistent with those
provided by SEM. This should be a valid approach at least at
the unit of analysis (a single hole) because the diameter of
each hole was calculated as the average value of ellipse major
and minor axes. The ellipse fitting algorithm has however a
small bias towards elliptical shapes [27], meaning that even
for a perfectly round shape the program tends to find a weakly
ellipsoidal shape due to its limited numerical precision. The
approach should also be valid on ensemble level (one field of
the OSS for measuring one hole and (2) use this value together
with an ensemble measurement to derive the calibration constant for an ensemble of holes occupying a large area.
In practice we relied on [24, 25] and the requirements
in section 2.1.1 and chose SEM as a base for comparison,
our gold standard. For this purpose we used a calibration specimen, figure 11, to determine the accuracy of the
SEM. The calibration specimen is a grid, the box of which
was measured three times, both vertically and horizontally
using the Mountains Map program’s distance measurement
features [26]. We defined the SEM measurement uncertainty
according to [24].
The uncertainty of D and d for each cavity measurement
was defined as required in [24]. The SEM calibration specimen
measurement’s uncertainty was combined with the d and D
measurement’s uncertainty values obtained with the SEM. The
9
A Karadzhinova et al
Meas. Sci. Technol. 25 (2014) 115403
Figure 20. The 44 outer cavities (D) in TS#2 measured with SEM and OSS before (BC) and after (AC) calibration.
5. Conclusion
view) because we were able to measure a single hole diameter
and calculate its correction and on cm2 scale, because we were
able to translate this measurement and obtain the same result
and correction for the same single hole.
This work is important to the field of metrology, as well
as for quality assurance and fabrication of GEM detectors.
For QA and fabrication of GEM it is important because the
size and shape of the holes influence their (and the detector’s)
gas multiplication factor. Improving the GEM foil fabrication
process is of major importance because it is assumed that the
characteristics of GEM foils strongly affect the behaviour of
the GEM foil detector. The achieved assurance rate, table 1,
means that the method could be useful in practice to check
large area structures, not only GEMs.
The most serious limitations with our general approach
to prove our claim comes from the fact that both methods
employed (OSS and SEM) essentially are 2D methods here
used to examine a 3D object with two diameters (d and D)
whose recorded values depend on maintaining fairly exactly
the focus distance control along the Z axis. Controlling focal
distance across large area scans (tight auto focusing) is nontrivial. Moreover, in the current OSS set-up we observe an axial
limitation (along the Z axis) due to limitations in camera pixel
size (1.73 µm), magnification of the optical system (1X) and
illumination wavelength ≈ 0.5 µm. Each of these factors also
affects the OSS image quality and consequently the accuracy of
the d and D estimates, which are at the core of the QA process.
We need to maintain tight autofocus during large fast
motion; the precision and accuracy of OSS depends on the
focus and this could be our most serious issue with our most
important result, i.e. why the main conclusion could be
challenged.
For more thorough OSS calibration we could use pieces
of GEM foils as TS and examine them with the OSS and
compare the results obtained with a 3D high-resolution noncontact imaging system. This could improve the current calibrated OSS set-up. Using 3D imaging would allow us to study
the GEM hole geometry, not only the GEM foil surface.
The OSS calibration was successful across a 950 × 950 mm2
area for the narrow range of D and d values present. A
calibration factor of 1.01 ± 0.01 for the inner cavities and
0.99 ± 0.01 for the outer cavities was applied as required by
ISO. The collected data possessed a 95% confidence limit
narrower than ± 1.03 µm as calculated in concordance with
UKAS M3003.
Acknowledgments
This work was partly supported by the EMRP NEW08
MetNEMS project and an associate research excellence grant
NEW08-REG3. The EMRP is jointly funded by the EMRP
participating countries within EURAMET and the European
Union.
We thank Mr Timo Rauhala and Doc. Marianna Kemell for
SEM support.
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